Junhyuk Lee

Efficient colorimetric pH sensor based on responsive polymer-quantum dot integrated graphene oxide.

In this paper, we report the development of a versatile platform for a highly efficient and stable graphene oxide (GO)-based optical sensor that exhibits distinctive ratiometric color responses. To demonstrate the applicability of the platform, we fabricated a colorimetric, GO-based pH sensor that responds to a wide range of pH changes. Our sensing system is based on responsive polymer and quantum dot (QD) hybrids integrated on a single GO sheet (MQD-GO), with the GO providing an excellent signal-to-noise ratio and high dispersion stability in water. The photoluminescence emissions of the blue and orange color-emitting QDs (BQDs and OQDs) in MQD-GO can be controlled independently by different pH-responsive linkers of poly(acrylic acid) (PAA) (pKa=4.5) and poly(2-vinylpyridine) (P2VP) (pKa=3.0) that can tune the efficiencies of Förster resonance energy transfer from the BQDs to the GO and from the OQDs to the GO, respectively. As a result, the color of MQD-GO changes from orange to near-white to blue over a wide range of pH values. The detailed mechanism of the pH-dependent response of the MQD-GO sensor was elucidated by measurements of time-resolved fluorescence and dynamic light scattering. Furthermore, the MQD-GO sensor showed excellent reversibility and high dispersion stability in pure water, indicating that our system is an ideal platform for biological and environmental applications. Our colorimetric GO-based optical sensor can be expanded easily to various other multifunctional, GO-based sensors by using alternate stimuli-responsive polymers.

Intraband conductivity response in graphene observed using ultrafast infrared-pump visible-probe spectroscopy

Graphene, a monolayer of carbon atoms arranged in a hexagonal pattern, provides a unique two-dimensional (2D) system exhibiting exotic phenomena such as quantum Hall effects, massless Dirac quasiparticle excitations and universal absorption&conductivity. The linear energy-momentum dispersion relation in graphene also offers the opportunity to mimic the physics of far-away relativistic particles like neutron stars and white dwarfs. In this letter, we perform a counterintuitive ultrafast pump-probe experiment with high photon energies to isolate the Drude-like intraband dynamics of photoexcited carriers. We directly demonstrate the relativistic nature of the photoexcited Dirac quasiparticles by observing a nonlinear scaling of the response with the density of photoexcited carriers. This is in striking contrast to the linear scaling that is usually observed in conventional materials. Our results also indicate strong electron-phonon coupling in graphene, leading to a sub-100 femtosecond thermalization between high energy photoexcited carriers and optical phonons.

Stimuli‐Responsive, Shape‐Transforming Nanostructured Particles

Development of particles that change shape in response to external stimuli has been a long-thought goal for producing bioinspired, smart materials. Herein, the temperature-driven transformation of the shape and morphology of polymer particles composed of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCPs) and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) surfactants is reported. PNIPAM acts as a temperature-responsive surfactant with two important roles. First, PNIPAM stabilizes oil-in-water droplets as a P4VP-selective surfactant, creating a nearly neutral interface between the PS and P4VP domains together with cetyltrimethylammonium bromide, a PS-selective surfactant, to form anisotropic PS-b-P4VP particles (i.e., convex lenses and ellipsoids). More importantly, the temperature-directed positioning of PNIPAM depending on its solubility determines the overall particle shape. Ellipsoidal particles are produced above the critical temperature, whereas convex lens-shaped particles are obtained below the critical temperature. Interestingly, given that the temperature at which particle shape change occurs depends solely on the lower critical solution temperature (LCST) of the polymer surfactants, facile tuning of the transition temperature is realized by employing other PNIPAM derivatives with different LCSTs. Furthermore, reversible transformations between different shapes of PS-b-P4VP particles are successfully demonstrated using a solvent-adsorption annealing with chloroform, suggesting great promise of these particles for sensing, smart coating, and drug delivery applications.

Normal and Abnormal Glow Discharge Modes in Atmospheric Plasmas

Summary form only given. By the optical observation as well as electrical measurements, transition of the Townsend, normal glow, and abnormal glow discharge modes were observed in an atmospheric pressure RF plasma produced in the ambient air with helium or argon supply gas. The plasma was produced between the two plate electrodes up to about 1 cm apart with the powered electrode connected to a RF source and a dielectric plate covered the ground electrode. In the normal glow discharge mode, discharge sustaining voltage and current density showed almost constant value of 165 V and 13 mA/cm2, respectively, and most of the applied RF power dissipated to enlarge plasma area. On the other hand, once the plasma covered the whole discharge area, the discharge voltage became larger as current increased. A distinctive difference between the two discharge modes was also observed in the measured phase angle between the discharge current and the applied voltage. During the normal glow discharge mode, increase of the input power brought about the phase angle change from 90deg to 50deg because of the plasma resistance drop by plasma area increase. In the abnormal glow mode, however, the phase angle augmented from 50deg to 60deg due to the plasma impedance change caused by shrink of the sheath width. The experimental results so far indicate that the discharge modes observed under the atmospheric pressure show basically same characteristics as those of low-pressure DC discharges

Plasma Characteristics Due to Helium and Oxygen Mixing in Argon-Based Atmospheric Plasmas

Summary form only given. Argon, helium, and oxygen gases are frequently used as a supply gas for generating stable plasmas and/or for aiding industrial applications of atmospheric plasmas. Since each gas demonstrates its own unique characteristics, choice of the gas may be important for a particular process due to the merits and drawbacks for the application. In this paper, results are presented first about the plasma characteristics based on the pure argon or helium plasmas produced in the ambient air, and then change of the characteristics brought about by mixing of He and O2 gases as an additive gas to the Ar plasmas. Electrical and optical spectroscopic diagnostics show the variation of plasma characteristics such as breakdown voltage, I-V curve, rotation temperature, and spatial uniformity of the plasma. In the case of 10 slm flow rate of Ar and He, respectively, both breakdown voltage (Vb) and rotational temperature (Trot) significantly decreased from 430 V to 300 V and from 470 K to 360 K. On the other hand, in the case of O2 (10 sccm) mixing in Ar (10 slm), Vb and Trot increased from 435 V to 450 V and from 490 K to 570 K. These changes can be explained by understanding their gas characteristics. In addition, increase of the additive gas amount resulted in deterioration of the spatial uniformity of the plasma in all cases. The experimental results illustrate that there exists a gas mixing ratio for optimum plasma condition, and it suggests that control of the atmospheric plasma characteristics can be possible to some extent by gas mixing for particular applications